Capacitive sensor systems are well known for their capability of providing control and operation of different appliances based on the presence of a human body. Such systems in general are for instance described in U.S. Pat. No. 4,453,112 and U.S. Pat. No. 5,621,290. In these documents a sensor electrode is arranged on the window frame of a car window. As soon as a portion of a human body, such as a hand, approaches the sensor electrode, the capacitance between the sensor electrode and an earth electrode increases. This increase in capacitance changes the frequency of an output signal of the electrode, which is compared to a reference, and a motor moving the window operates based on this change. The sensor system can respond to semi-conductive elements such as a human body, but plastics and wood will not cause any effect.
The not yet published Swedish patent application SE 0402261-2 discloses a capacitive sensing system provided to solve the problem of detecting a small body part between the door and the body of a refrigerator cabinet equipped with a capacitive sensing system. Normally, the cabinet body will make the door antenna blind since its major influence on the capacitance is much larger that the smaller influence on the capacitance caused by the body part. To solve this, the disclosed invention comprises an electromagnetic shield between the door and the fridge body, mounted like a gasket at the edge of the cabinet body. The shield should remove the influence caused by the cabinet body. This shield has fixed position relative to the fridge body and a constant capacitance in relation to it. The door is electrically connected to this shield and the movement of the door process does not change the door capacitance to the fridge body.
However, one disadvantage with the disclosed system is the high construction cost. This frame should be thin conductive foil (cupper for example) but from aesthetic point of view it should be covered by some plastic elements and all sides of this frame should be electrically connected to each other and to the control board. When closed the door cover frame elements influence the gasket with big pressure and plastic need to be hard enough (expensive) to survive in this conditions.
Another problem is that the frame is not a real shield. The size of the door (see FIG. 1) is much bigger than the total area of the frame. This means that an electromagnetic field C3 is formed between the door and the fridge body. This will add an extra, varying capacitance to the measurement circuit of the system. In fact, the shield (since it is not a real shield) could be replaced by “normal” constant capacitor C1 located on the control board. Then we are back to a capacitive sensing system for a refrigerator cabinet without a shield. The capacitance C3 will depend on the door angle and influence the total capacitance of said circuit (see FIG. 2). Moreover, because the frame is much closer to the fridge body, its capacitance C1 is much higher (about 2-3 nF and it's mean 10 times bigger than the human body to door's capacitance, C2).
We will therefore have not only one constant capacitance of the shield to body, but two capacitors C1,C3 connected in parallel—frame to the fridge and door to the fridge with a total capacitance 10 times higher than human body to door (C2). FIG. 1-2 illustrates this. Moreover, we have to deal with the C3 variances because of the door angle. Because of this we will need a high-resolution ADC (dynamic range about 20 000) and complicated signal processing with amplifying only a part of a signal within full range to be able to detect a human body movement. The problem will be to identify if a variation in total capacitance depends on a door movement or a human body presence.